Water-in-Ionic Liquid Microemulsion Formation in Solvent Mixture of Aprotic and Protic Imidazolium-Based Ionic Liquids

We report that water-in-ionic liquid microemulsions (MEs) are stably formed in an organic solvent-free system, i.e., a mixture of aprotic (aIL) and protic (pIL) imidazolium-based ionic liquids (ILs) containing the anionic surfactant dioctyl sulfosuccinate sodium salt (AOT). Structural investigations using dynamic light, small-angle X-ray, and small-angle neutron scatterings were performed for MEs formed in mixtures of aprotic 1-octyl-3-methylimidazolium ([C₈mIm⁺]) and protic 1-alkylimidazolium ([C_<i>n</i>ImH⁺], <i>n</i> = 4 or 8) IL with a common anion, bis­(tri­fluoro­methanesulfonyl)­amide ([TFSA^–]). It was found that the ME structure strongly depends on the mixing composition of the aIL/pIL in the medium. The ME size appreciably increases with increasing pIL content in both [C₈mIm⁺]­[TFSA^–]/[C₈ImH⁺]­[TFSA^–] and [C₈mIm⁺]­[TFSA^–]/[C₄ImH⁺]­[TFSA^–] mixtures. The size is larger for the <i>n</i> = 8 system than that for the <i>n</i> = 4 system. These results indicate that the shell part of MEs is composed of both AOT and pIL cation, and the ME size can be tuned by pIL content in the aIL/pIL mixtures

Structural Study on Magnesium Ion Solvation in Diglyme-Based Electrolytes: IR Spectroscopy and DFT Calculations

We investigated the solvation structure of Mg ions in a diglyme (G2)-based electrolyte solution for Mg ion batteries. The Walden plots based on ionic conductivity and viscosity of the Mg­(TFSA)2/G2 [TFSA: bis­(trifluoromethanesulfonyl)­amide] solutions indicated that the dissociativity of Mg­(TFSA)2 gradually increased, even with increasing salt concentration (cMg). This behavior is similar to that of the analogous triglyme (G3)-based solutions. Infrared (IR) spectroscopy revealed that Mg ions were coordinated by two G2 molecules to form an octahedral [Mg­(G2)2]2+ complex in the cMg range examined herein (≤0.92 M). The detailed coordination geometry of the [Mg­(G2)2]2+ complex was evaluated using density functional theory calculations. We found that G2 molecules coordinated in a tridentate ligand fashion to form an octahedral [Mg­(tri-G2)2]2+ complex. This result was different from that of the G3 system; i.e., G3 molecules acted in three ligand modes (bidentate, tridentate, and tetradentate) such that multiple solvation complexes such as [Mg­(tri-G3)2]2+ and [Mg­(bi-G3)­(tetra-G3)]2+ complexes were formed. This difference between the G2 and G3 systems might be related to an entropy contribution in the liquid state; i.e., only one coordination structure exists for [Mg­(tri-G2)2]2+ in the G2 system, whereas more coordination complex structures can be formed in the G3 system

Kinetic Aspect on Gelation Mechanism of Tetra-PEG Hydrogel

We carried out a kinetic study on the gelation reaction of AB-type cross-end coupling of two tetra-arm poly­(ethylene glycol) (Tetra-PEG) prepolymers having amine (Tetra-PEG-NH2) and activated ester (Tetra-PEG-NHS) terminal groups by ATR-IR and UV spectroscopies. The reaction rate constant for the gelation of Tetra-PEG, kgel, was determined in aqueous solutions with varying both prepolymer volume fraction, ϕ, and molecular weight, Mw, of the prepolymers. It was clearly found that the value of kgel is independent of both ϕ and Mw, and is comparable to that of the corresponding linear-PEG system. The kgel value is obtained to be around 70 dm3 mol–1 s–1, which is much smaller than the reaction rates of typical diffusion-controlled reaction (e.g., 108–109 dm3 mol–1 s–1) and of cross-linking photopolymerization (104–105 dm3 mol–1 s–1). From these results, we concluded that the gelation reaction of Tetra-PEG gel is not diffusion-limited but reaction-limited process, i.e., the diffusion motion is much faster than the reaction rate. It is thus expected that Tetra-PEG prepolymer chains can diffuse in the solution during gelation process, leading to homogeneity and high-strength of Tetra-PEG gel. These discussions imply that, in order to achieve high-efficient and homogeneous gel, it is necessary to choose reaction groups so as to undergo reaction-limited reaction

2,2,2-Trifluoroethyl Acetate as an Electrolyte Solvent for Lithium-Ion Batteries: Effect of Weak Solvation on Electrochemical and Structural Characteristics

We report the characteristics of 2,2,2-trifluoroethyl acetate (TFEAc) as a new type of electrolyte solvent for lithium (Li)-ion batteries. TFEAc-based electrolyte solutions containing 1.0 mol dm–3 LiTFSA salt [TFSA: bis­(trifluoromethanesulfonyl)­amide] exhibited a Li-ion insertion reaction into the negative graphite electrode in the first cycle; however, the performance was noticeably degraded during subsequent cycles due to the lack of solid electrolyte interphase (SEI) formation on the electrode. The electrode reaction was markedly improved when a small amount of ethylene carbonate (EC) was added into the LiTFSA/TFEAc solution, which demonstrated a high-rate charge/discharge performance superior to that of the conventional carbonate-based Li-ion battery electrolyte, that is, 1.0 mol dm–3 LiPF6 in the EC + dimethyl carbonate mixture. Quantitative Raman spectral analysis and density functional theory calculations revealed that TFEAc could be categorized as an organic solvent with low solvation power (i.e., with a predicted Gutmann donor number of 9.1); thus, Li ions mainly formed contact ion-pair complexes, [Li­(TFEAc)2(TFSA)], in binary LiTFSA/TFEAc solutions. Adding EC into the TFEAc electrolyte modified the Li-ion complex structure; namely, Li ions were coordinated with each TFEAc, EC, and TFSA component to yield [Li­(TFEAc)­(EC)­(TFSA)] as the major species, which coexisted with the ion pair [Li­(TFEAc)2(TFSA)]. We discuss the effect of the weak coordination solvent and EC additive on the graphite electrode reaction from the aspects of Li-ion desolvation and SEI formation

Preparation and Hydrophilicity/Lipophilicity of Solubility-Switchable Ionic Liquids

Various solubility-switchable ionic liquids were prepared. Their syntheses were readily achieved in a few steps from glyceraldehyde dimethylacetal or its derivatives. Pyridinium, imidazolium, and phosphonium derivatives also exhibited solubility-switchable properties; acetal-type ionic liquids were soluble in organic solvents, while diol-type ones exhibited a preference for being dissolved in the aqueous phase. The solubility of the ionic liquids prepared in this study also depended on the number of carbon atoms in the cationic parts of the ionic liquids. Interconversion between the diol-type and the acetal-type ionic liquids was readily achieved under the standard conditions for diol acetalization and acetal hydrolysis. One of the prepared ionic liquids was also examined as a solvent for an organic reaction

Nearly Ideal Polymer Network Ion Gel Prepared in pH-Buffering Ionic Liquid

We report a high-toughness ion gel with a nearly ideal polymer network prepared in an imidazolium-based aprotic ionic liquid (aIL) with a controlled solution pH. We formed the ion gel from tetra-armed poly­(ethylene glycol) (TetraPEG), i.e., an A–B-type cross-end coupling reaction of two different TetraPEG prepolymers. To complete this A–B-type reaction, we needed to optimize the reaction rate such that the two TetraPEGs were mixed homogeneously, which strongly depends on the pH or [H⁺] in the aIL solution. To control solution pH, we established a “pH-buffering IL” by adding an imidazolium-based protic IL (as a proton source) and its conjugated base to the solvent aIL. We demonstrated that the pH-buffering IL exhibits a successful pH-buffering effect to maintain a constant pH (≈16.2, apparent value) during the gelation reaction. From a kinetic study, we found that the gelation reaction undergoes a simple second-order reaction of the two TetraPEGs in the pH-buffering IL. The gelation rate constant, <i>k</i>_gel, in the present ion gel system was 2 orders of magnitude smaller than that in the corresponding hydrogel system, which is ascribed to the difference in the activation entropy, Δ<i>S</i>^‡, of the cross-end coupling reactions. The reaction efficiency at the cross-linking point was experimentally estimated to be 92% by spectroscopic measurements. We thus conclude that a nearly ideal polymer network was formed in the pH-buffering IL system. This is reflected in the excellent mechanical property of the ion gel, even at a low polymer content (=6 wt %)

Liquid Structure and Preferential Solvation of Metal Ions in Solvent Mixtures of<i> N</i>,<i>N</i>-Dimethylformamide and <i>N</i>-Methylformamide

Raman spectra of aprotic N,N-dimethylformamide (DMF) and protic N-methylformamide (NMF) mixtures containing manganese(II), nickel(II), and zinc(II) perchlorate were obtained, and the individual solvation numbers around the metal ions were determined over the whole range of solvent compositions. Variation profiles of the individual solvation numbers with solvent composition showed no significant difference among the metal systems examined. In all of these metal systems, no preferential solvation occurs in mixtures with DMF mole fraction of xDMF xDMF > 0.5. The liquid structure of the mixtures was also studied by means of small-angle neutron scattering (SANS) and low-frequency Raman spectroscopy. SANS experiments demonstrate that DMF molecules do not appreciably self-aggregate in the mixtures over the whole range of solvent composition. Low-frequency Raman spectroscopy suggests that DMF molecules are extensively hydrogen-bonded with NMF in NMF-rich mixtures, whereas NMF molecules extensively self-aggregate in DMF-rich mixtures, although the liquid structure in neat NMF is partly ruptured. The bulk solvent structure in the mixtures thus varies with solvent composition, which plays a decisive role in developing the varying profiles of the individual solvation numbers of metal ions in the solvent mixtures

Solvation of Magnesium Ion in Triglyme-Based Electrolyte Solutions

We report here structural study on solvation of Mg²⁺ ion in triglyme (G3)-based solutions applying as a novel electrolyte for rechargeable Mg batteries. In Mg­(TFSA)₂/G3 electrolyte solutions (TFSA = bis­(trifluoromethanesulfonyl)­amide), we found from Raman spectroscopy that Mg²⁺ ion is solvated with two G3 molecules to form [Mg­(G3)₂]²⁺ complexes. No direct coordination of TFSA^– anion to Mg²⁺ ion occurs in the solutions with the salt concentrations <i>c</i>_Mg = 0–1.60 M. The geometries and interaction energies for the [Mg­(G3)₂]²⁺ were evaluated by DFT calculations and indicated that G3 molecules in the most stable complex act as a tridentate ligand, i.e., octahedral [Mg­(tri-G3)₂]²⁺. However, the Raman spectra implies that [Mg­(tri-G3)₂]²⁺ coexists with [Mg­(tetra-G3) (bi-G3)]²⁺ in the solutions where tetra-G3 and bi-G3 are G3 molecules acting as tetra- and bidentate ligands, respectively, in the solvation sphere. The Walden plots indicated that the dissociativity (or ionicity) of Mg­(TFSA)₂ in G3 solutions increases with increasing <i>c</i>_Mg, which is opposite to conventional organic electrolyte solutions but is similar to the LiTFSA/glyme solutions

Structural and Electrochemical Properties of Li Ion Solvation Complexes in the Salt-Concentrated Electrolytes Using an Aprotic Donor Solvent, <i>N</i>,<i>N</i>‑Dimethylformamide

We report the relation between the structural and electrochemical properties of <i>N</i>,<i>N</i>-dimethyl­formamide (DMF)-based electrolytes containing lithium bis­(trifluoro­methane­sulfonyl)­amide (LiTFSA) in the concentration range <i>c</i>_Li = 0–3.2 mol dm^–3. Raman spectroscopy and DFT calculations indicate that Li⁺ ions are solvated by DMF molecules in the form of [Li­(DMF)₄]⁺ complexes at low <i>c</i>_Li (<2.0 mol dm^–3, LiTFSA:DMF = 1:4 by mol), whereas the coordination of TFSA^– anions to Li⁺ ions occurs and gradually increases as <i>c</i>_Li increases above 2.0 mol dm^–3. Quantitative Raman data analysis reveals that TFSA^– anions coordinate with Li⁺ ions in a monodentate manner (mono-TFSA) in the <i>c</i>_Li range of 2.0–2.5 mol dm^–3, and mono-TFSA coexists with TFSAs as a bidentate manner (bi-TFSA) in solutions with <i>c</i>_Li > 2.5 mol dm^–3. The high <i>c</i>_Li solutions, in which all the DMF molecules solvate to Li⁺ ions (i.e., no DMF remains in the bulk), make the electrochemical window wider; the oxidative stability is enhanced owing to lower HOMO energy levels of solvated DMF molecules relative to those in the bulk. The salt concentration also controls the reductive stability; coordinated TFSA^– anions within the Li-ion complexes formed in concentrated solutions affect the LUMO energy levels of the electrolyte. The LUMOs located on the TFSA^– anions lead to a preferential reduction of the TFSA component rather than DMF to form a solid electrolyte interphase on graphite negative electrodes, resulting in the Li-ion insertion/desertion into/from graphite in the concentrated solutions